feat: play a track's input through as it arrives

[HenryMBaldwin]

• Add a per-track passthrough setting on the fourth settings column.
• Mix each distinct input once however many tracks ask for it, at the loudest of their levels.
• Pass through ahead of the pause, so a frozen transport still lets you hear yourself.
• Fade on and off over the declick length.
• Save the setting with the track.

Unity and centred: a mono input lands on every output channel, a pair on left and right. Nothing is delayed to line up with the loops.